Multimode light is an integral part of the pumping element of fiber lasers and amplifiers. A tapered fiber bundle as shown in FIG. 1 is one exemplary configuration used to combine multiple pump sources (each source providing light that is propagating along a separate fiber) into a structure that provides pump light output within a diameter of most fibers. As shown, the individual fibers (with nominal dimensions) are collected into a group that is defined as a “bundle” (a cross-section of the bundle shown in FIG. 1). The bundle is then heated and tapered so as to essentially meld into a single fiber (cross-section also shown in FIG. 1). This tapered section is capable of providing the combined pump lights as the output of the bundle.
In use, a separate optical fiber is typically spliced to the endface of the taper and used to deliver the total, combined pump signal to an associated optical fiber amplifier or laser. The configuration of the tapered fiber bundle and the output fiber is often referred to as a “pump combiner”, or simply a “combiner”. The output fiber has both core and cladding, where in some cases the cladding comprises glass.
As a matter of course, there is always a desire to increase the transmission efficiency of the light propagating through this type of combiner from the tapered bundle to the output fiber (or, more generally, from any type of “cladding-less” fiber to a cladded optical fiber). Indeed, higher transmission efficiency improves the overall system efficiency and also reduces the power requirements associate with high power fiber lasers and amplifiers (e.g., greater than 2 kW). Also, higher pump transmission efficiency reduces the loss component of the system, since the light energy associated with transmission loss is converted to heat and reduces system reliability. Hence, higher multimode transmission efficiency is desirable.
The power threshold for nonlinear effects in fiber lasers and amplifiers is inversely proportional to the fiber length. The required gain length is itself proportional to the cladding area of the doped fiber. In order to minimize nonlinearities, the smallest possible cladding diameter is desirable. On the other hand, the amount of pump power that can be coupled into the gain fiber is proportional to its cladding area; thus, a large cladding area is desirable for this reason. Since the numeral aperture (NA) of the light that can propagate through a tapered fiber bundle increases with taper ratio, the NA at the output of the taper should not exceed the NA of the output fiber (important in order to achieve low loss). This condition is established by a “brightness” condition for tapered bundles of multimode fibers defined as follows:Dout2NAout2≧n˜Din2NAin2,where Dout is the diameter of the output fiber, NAout is the numerical aperture of the output fiber, “n” is the number of fibers in the bundle, Din is the diameter of a pump input fiber and NAin is the numerical aperture of the pump input fiber.
The limitations of available pump diode brightness do not allow for efficient pumping of small diameter gain fibers, due to the higher taper ratio required for use with the smaller gain fiber. A high taper ratio increases the NA of light and, therefore, decreases brightness and increases pump losses (decreasing the maximum power available at the output of the combiner).